(a) Field of the Invention
The present invention relates to resin compositions which contain bismaleimide compounds and can be cured by application of heat or irradiation of light, and to films and laminates produced by using them, particularly to novel resin compositions which are rendered low temperature-thermosetting and photo-curing by combining bismaleimide compounds with novel accelerators, and are particularly useful for the production of laminates and printed wiring boards. The present invention also relates to the production of multilayer wiring boards by a build-up process using such resin compositions.
(b) Description of the Related Art
In the production of laminates and printed wiring boards, thermosetting resins including epoxy resins, phenolic resins, melamine resins and bismaleimide compounds are extensively used as insulating materials. Among all, bismaleimide compounds render particularly high-heat resistance, and are common main ingredients in insulating materials of a high-heat resistance grade.
In practical applications, bismaleimide compounds are cured by dimerization utilizing the splitting of unsaturations or by crosslinking through the Michael addition of active amines. An exemplar literature is "The Newest Heat Resistant Resin", Edit. Satoru Mita, p42, Bismaleimide resins (Sogo Gijutsu Center Co., Ltd., May, 1987).
Particularly, in the industries of insulating substrates and laminates, bismaleimide compounds are marketed as polyimide laminating materials in combination with modifiers, such as epoxy resins. Because of their excellent heat resistance, they are used for applications requiring high reliability, such as wiring boards for super computers or airplanes.
The double bonds contained in bismaleimide compounds, however, have low reactivity, requiring heating for a long time at high temperatures to thermoset. Thus radical initiators, such as peroxides, are generally added, even which require heating for a long time at high temperatures, causing the warp and contraction of substrates.
There are also an increasing demand for photo-curing insulating materials for solder resist, and it has been tried actively to use photosensitive resins not only as the conventional surface insulating materials, such as solder resist, but also as interlayer-insulating or -connecting materials.
For example, common multilayer wiring boards are produced by superposing sequentially an insulating substrate bearing an internal circuit layer, prepregs prepared by impregnating glass fabric with an epoxy resin followed by semi-cure, and a copper foil, pressing them with heat, drilling to make through holes for interlayer connection, electroless-copper-plating the copper foil surface and the internal walls of the through holes, optionally electro-copper-plating to a thickness required of circuit conductor, and removing unnecessary copper. As electronic instruments have been downsized, lightened and made multifunctional increasingly, LSIs and chip components are also increased in integration, accompanied by rapid changes in shape, such as downsizing and increased pins. This accelerates the fine wiring of multilayer wiring boards and increases the packaging density of electronic components. However, there is a limit in decreasing wiring pitch with the current techniques, which permit wiring pitches of 75 to 100 .mu.m for mass production. This makes it difficult to significantly increase the wiring density by mere reduction of wiring pitch. The improvement in wiring density is also inhibited by through holes which occupy an area of about 300 .mu.m diameter per hole. Through holes have such a large diameter since they are generally made by mechanical drilling, narrowing room for wiring design. As a means for solving these problems, Japanese Patent Examined Publication No. 4-55555 and Japanese Patent Unexamined Publication No. 63-126296 disclose photo-processing methods of making via holes for interlayer connection in photosensitive insulating resin layers provided on insulating substrates.
Conventional thermosetting materials themselves, however, have no photo-curability and inapplicable for the above purpose. To the contrary, most of conventional photosensitive materials of resist have enough photo-curability, but are hardly applicable to the above purpose due to lack of the properties of insulating materials, particularly heat resistance and moisture resistance.
According to a known attempt to make the conventional thermosetting materials photo-curable, epoxy resins are combined with photo-cation precursors. The photo-sensitivity of this system, however, is so poor as to require a large light energy even to induce insufficient cure reaction. Thus it is the fact that the properties of cured products are considerably inferior to thermoset products. So this technique has found little applications, particularly in applications requiring fine patterning, though long years have passed after its publication.
In multilayer wiring boards, the thin insulating layers 50 to 100 .mu.m thick need to be composed of materials having good heat resistance. Therefore, bismaleimide compounds are useful thermosetting materials of insulating layers to impart good heat resistance. Bismaleimide compounds, however, are too poor in reactivity to polymerize or dimerize them by using photo-polymerization initiators, and have been added to photo-cure systems merely as thermosetting ingredients.
As described above, the cure reactions of thermosetting materials involve problems. Particularly, both the thermosetting and photo-cure reactions of bismaleimide materials involve problems, so that bismaleimide materials have found little applications in the fields of laminates and printed wiring boards, except for the use as imide laminate materials.